When designing and constructing a massive concrete structure, such as a concrete dam, a foundation for a main tower or an anchorage for a suspension bridge, a major concern relates to a rise in temperature of the structure.
Temperature variation of the structure is caused by a combined effect of (a) heat generation by hydration of cement and (b) seasonal and daily change in temperature of the atmospheric air. In particular, during construction of such a structure, hydrating heat may cause a temperature rise of about 15.degree. to 25.degree. Celsius. Most of the stresses to which the concrete structure under construction is subjected during such a temperature rise are compressive stresses. However, because the concrete is still young, the levels of these compressive stresses are negligibly small and, therefore, the modulus of elasticity is also small and the concrete shows creep behavior.
After reaching its peak temperature, the concrete structure starts to cool down. Since the concrete has developed reasonable elastic properties by that time, a high level of tensile stress is observed in the concrete due both to a constraint by a rock foundation (which is called a "foundation constraint effect" or an "external constraint effect") and to a constraint by the concrete itself (which is called an "internal constraint effect"). On the other hand, the temperature of the underlying concrete is equal to the atmospheric temperature and, therefore, the temperature distribution of a completed large-size concrete structure exhibits a periodic profile in its vertical direction, because of the long time required to complete such a large-sized structure, and because the atmospheric temperature naturally varies periodically during construction of the structure.
For example, yearly variation of air temperature in Japan is .+-.12.degree. Celsius, or more. This periodic temperature change, coupled with a reduction in temperature due to the hydration heat after the concrete structure has reached its peak temperature, may produce cracks in the massive concrete structure. Once a crack develops in the concrete, it may propagate suddenly or cause additional cracks, thus causing great damage to the structure.
Accordingly, several techniques have been developed and used for controlling the temperature of a massive concrete structure or for preventing crack initiation and propagation therein. Any such a technique must be applied with due consideration of the size, type, period and method of construction of the structure, the duration of the construction, the thermal properties of the concrete used therein, and the weather conditions at the construction site. In particular, one or more of the following techniques are typically utilized:
(1) Where the amount of heat which will be generated and stored in the placed concrete is estimated correctly, an adequate construction method may be used for suppressing the temperature rise as much as possible. For example, one may reduce the quantity of cement used, may place concrete in half lifts, or may use a pipe cooling or pre-cooling technology. PA1 (2) A contraction joint may be provided in advance, at locations where cracks are projected to develop. PA1 (3) A proper design or construction procedure may be used to prevent crack initiation in the concrete. For example, the concrete layer adjacent the rock foundation, which will be strongly constrained by the rock, may be placed in a season having relatively lower temperatures. Moreover, the placed layer of concrete, when still at a height level lower than the final design height, should not be exposed for a long time. Further, the surface of the placed concrete should be adiabatically cured to prevent a rapid temperature change. Furthermore, the concrete can be placed with appropriate consideration of its creep characteristics.
The foregoing techniques must be used selectively in order to prevent crack initiation and propagation. Accordingly, it is very important to determine correctly the temperature distribution occurring during and after construction of the massive concrete structure.
In that regard, various methods have been proposed to determine temperature distributions and the thermal stresses corresponding thereto. It is a common recent practice to apply a numerical analysis, such as a finite element or boundary element method, which takes into consideration heat generation by hydration and a nonlinear behavior due to concrete hardening. However, a conventional design method, using a concept of constraint factor, continues to be in common use for determining a stress vertical distribution. In particular, this method is widely used for outline design of the concrete structure because, in such design, numerous design cases must be examined.
Stress may be estimated by using the concept of constraint factor, as follows: Let R be a constraint factor of a rectangular concrete block placed on a rock foundation. Stress is determined as a function of the geometry and material properties of both of the massive concrete structure and of the rock foundation. The factor is typically given in the form of a graph or a chart. Then, the horizontal stress .sigma. is calculated as EQU .sigma.=R E.sub.c .alpha..sub.c .DELTA.T,
where E.sub.c is the elastic modulus of concrete, .alpha..sub.c the thermal expansion coefficient of concrete, and .DELTA.T the temperature change, which has typically been estimated as .DELTA.T=T.sub.r +Ta-Tm, where T.sub.r is a reduction in the concrete temperature after the peak temperature, T.sub.m the mean air temperature and Ta is the amplitude of seasonal change of the mean air temperature.
This procedure for stress estimation is based on an assumption that a rectangular-shaped concrete block is placed on a semi-infinite layer, and that a uniform temperature drop AT will occur in the block. Therefore, with this procedure it is impossible to calculate the stress that will occur in each layer formed under an atmospheric temperature which changes from season to season.
In view of the difficulties of the prior art, it is thus important to establish a simple and accurate method for analyzing the stress that will occur in each of the concrete layers due to variations in the atmospheric temperature. Such a method can then be used to develop an optimal method for designing a desired concrete structure and determining an appropriately corresponding construction sequence.
It is further noted that conventional cooling procedures such as pipe cooling and pre-cooling, which intend to keep the concrete temperature under a certain level in order to prevent cracks from developing in concrete, are not efficient. Pipe cooling, which uses water pipes to remove heat, is not cost-effective because of the necessity to cool all the concrete layers uniformly. Such uniform cooling requires a large-scale cooling apparatus. Pre-cooling, which forces cooling down of concrete materials before mixing, makes it difficult to control the concrete temperature before placing because the concrete materials become warmed during transportation to the construction site, so that pre-cooling also is not cost-efficient.
It is accordingly an object of the invention to overcome the difficulties of the prior art and to provide a novel method for placing concrete in design and construction of massive concrete structures.
It is a more particular object of the invention to provide a method for partial heating or cooling of portions of a massive concrete structure during construction, in order to prevent formation of cracks.